The present invention relates to an elongated percussive rock drilling element comprising at least one thread portion and a flush channel.
During percussive rock drilling, shock waves and rotation are transferred from a drill machine via one or more elongated elements in the form of so-called drilling steels or drilling rods to a cemented carbide equipped drill bit. The percussion frequency is typically from about 50 to about 70 Hz. The material in such an element is subjected to corrosive attack during the drilling. This applies in particular to underground drilling where water is used as flushing medium supplied through the flush channel of the element and where the environment in general is humid. The corrosive attacks are especially serious in the most stressed parts, i.e., at the two ends of such an element, where the forces from the drill machine are directly or indirectly transferred to the element and from the element to the drill bit, respectively. In combination with pulsating stress, caused by bending stresses and the shock waves mentioned above, so-called corrosion fatigue arises, which means that cracks are created in the element through corrosion in combination with said stress, whereupon these propagate through the element until a rupture of the element occurs.
Accordingly, such a rupture normally occurs at the ends of the element, where mostly a thread portion is arranged for obtaining a power transmitting connection.
As a consequence of the above problem of corrosion fatigue, it has in U.S. Pat. No. 6,547,891 been suggested to make at least said thread portion of an element mentioned above of a corrosion resistant steel having a structure with a martensite content of greater than about 50 wt-% and less than about 100 wt-%,
in which the steel comprises 0.1 wt-% less than or equal to about C+N less than or equal to about 0.8 wt-% and Cr greater than or equal to about 11 wt-% or
Cr greater than or equal to about 5 wt-%, Mo less than or equal to about 5 wt-%, W less than or equal to about 5 wt-%, Cu less than or equal to about 2 wt-%, Mo+W+Cu greater than about 0.5 wt-% or
Cr+3.3 (Mo+W)+16N greater than about 10 wt-%.
Mechanical strength and core hardness required for the application of percussive rock drilling are obtained through the martensite structure of the matrix of the steel. However, the corrosion resistance is here of particular importance, which is obtained by the composition of the steel mentioned above and through which a passivation layer is formed on the surface, which prevents corrosion or reduces the corrosion rate and by that the corrosion fatigue. However such a drilling steel has a restricted life time in the form of the number of meters which may be drilled in a rock by this before it has to be rejected, which normally is necessary through a rupture in the region of a thread portion caused by mechanical fatigue. This means that there is an ongoing attempt to prolong this life time by such means that results in a total saving of costs.